1. Field of the Invention
The present invention relates to a video cassette recorder having a video index search system function (hereinafter VISS) and, more particularly, the present invention relates to a circuit for preventing image search malfunction from occurring due to variation in the speed of the video tape in which the normal VISS operation is carried out using a variable pulse width supplied by a Schmitt trigger circuit in accordance with variation of the speed of the video tape.
Korean Patent Application No. 93-396 is incorporated herein by reference for all purposes.
2. Description of the Prior Art
The VISS function in a video cassette recorder (VCR) is provided for easily searching the contents of a tape for recorded data by recording index information. This index information is generally provided utilizing variations in the duty of a control pulse, which is used for controlling the phase of the VCR capstan motor.
However, VISS requires overwriting of the tape for recording the index information. If signals within the existing control track are not completely erased, or if a tape is a worn-out tape in use for a long time, residual signals or noises are included in the index information.
Therefore, when VISS operations such as intro scan and index search are carried out, the gains of the residual signals or noises are sufficiently high in accordance with the increase in the speed of the video tape, with the result that a malfunction occurs. In a worst case, phase control of the capstan motor becomes impossible even during a search mode. This phenomenon will be described in further detail below.
It will be noted that in carrying out the VISS function, the user arbitrarily records the index information on the control pulse, which is used as a comparison signal, by employing the variation of the duty cycle of the control pulse in order to control the phase of the capstan motor.
Later, when the VISS operation is carried out, e.g., when searching for the recorded portion, the index information is picked up by detecting the duty cycle variation.
During the search mode, the reproduction operation is carried out starting from the relevant portion, while, during the intro scan mode, reproduction is carried out for 5 seconds and then fast forward winding (FF) or rewinding (REW) of the tape is carried out until the next index information is located.
As shown in FIG. 1A, during a normal mode of operation, a ratio of the duty of the control phase is maintained at about 60%, and reproduction is made at 30 Hz, so that the control signal can be used as a comparison signal for controlling the phase of the capstan motor.
When performing a VISS overwriting operation, as shown in FIG. 1B, the index information is recorded by utilizing the negative side of the control signal, so that a ratio of the duty of the control pulses should be about 30%. During the VISS index search/intro scan operations, the duty cycle variation of the relevant portion is detected to carry out the next operation.
When carrying out the VISS overwriting operation, the negative side of the control pulses which are recorded on the control track may not be erased sufficiently, with the result that residual signals R remain thereon. Particularly in the case of a worn-out video tape, such residual signals remain even on the positive side of the control pulse as noise N, thereby aggravating the problem.
The VHS specification requires that the ratio of the level of the main signals to the level of the residual signal should be above about 20 dB, but this is the condition for recording. When the fast forward or rewinding operations are carried out in a reproduction mode, the pulse width of the level of the noise N and the level of the residual signal R exceed the width of the Schmitt trigger pulse. This phenomenon is more significant at the beginning and the end of the tape.
When the VISS operation is carried out by fast forward winding or rewinding operations, the level of the noise N or the level of the residual signal R exceeds the pulse width of the Schmitt trigger circuit in the same way as in normal reproduction, with the result that a VISS malfunction occurs. That is, the residual signals come to exceed the pulse width of the Schmitt trigger circuit and, consequently, an output of the Schmitt trigger circuit having a duty which does not fit the VISS format is formed, thereby causing the VISS malfunction. See FIG. 1C.
U.S. Pat. No. 5,021,896, entitled FAST SEARCH DEVICE IN MAGNETIC RECORDING AND REPRODUCTION DEVICE, discloses a technique for controlling the relative speed between the rotary drum and the tape at a predetermined speed even during a search start condition, which condition occurs only momentarily, so that the information on the tape can be read even during a high speed search.
According to U.S. Pat. No. 5,021,896, when a high speed search command is received, the rotary drum is driven in such a manner that the relative speed becomes a predetermined speed without an increase in the tape speed. Furthermore, after the relative speed equals the predetermined speed, the speed of the rotary drum gradually varies so as to be suitable for a high speed search. On the other hand, the rotary speed of the tape reel is varied while maintaining the tape speed relative to the predetermined speed, until the speed of the tape reel reaches the desired value. Thus, owing to such arrangement, when the search starting (momentary) condition is initiated, the relative speed between the rotary drum and the tape is controlled at a predetermined speed, thereby making it possible to read the information on the tape.
While using the above described technique, to prevent such malfunction as searching the position of another music for a designated music or skipping the designated position is possible, it is impossible to prevent the malfunction due to the noise or residual signals.